1. Field of the Invention
The present invention relates to non-invasive observation of turbulent flow. More specifically, the present invention relates to transmitting a laser beam through a turbulent flow and temporally filtering the laser beam to provide data that includes useful information regarding the turbulence and the turbulent spectrum of the flow in two dimensions.
2. Description of Related Art
The nature of turbulence is such that great difficulties arise in measurement and observation of turbulent flows and turbulent heat conduction. A turbulent flow may be defined, in a flow of a fluid such as water or air, as a flow in which the local velocities and pressures fluctuate irregularly in a somewhat random manner. Turbulent heat conduction may be defined as the conduction of heat in a fluid by lateral and vertical eddy diffusion, with currents and eddies in a somewhat random manner.
Turbulence affects many events of interest. For example, an airfoil of an aircraft wing may designed to minimize turbulence induced drag, while simultaneously providing lift and maneuverability. The hull of a boat or a submarine may be designed to minimize turbulence. For many uses, including engineering design and real time flow monitoring, observation of turbulent flow is very useful, particularly if the flow can be monitored in real time. Furthermore it is useful if observation occurs non-invasively; i.e., without affecting the properties of the turbulence.
Present theory is incapable of accurately predicting turbulent effects; thus, experimental observation is vital. However, the expense of experimental observation can be large; a full-scale prototype may need to be built. As an example in aeronautics, turbulent effects are not fully appreciated until a prototype aircraft is built and tested in operation-an often expensive and dangerous process. If an accurate method for observing turbulence were available, the aircraft's design could be more fully tested with a low cost model, which would save a great deal of expense and time.
Systems have been developed for monitoring turbulence in a fluid flow. These systems include the standard "hot wire" technique, and Schlieren techniques.
The hot wire technique, which has been known for many years, physically positions a wire in the fluid flow. As fluid flows past the wire, it is cooled according to the local flow conditions. By applying a voltage and monitoring the wire with an ohmmeter, a measure of the amount of cooling can be obtained, which is related to the turbulence. The hot wire technique has many drawbacks, including the fact that the wire itself perturbs the flow; it is an invasive technique. Other drawbacks of the hot wire technique include poor spatial resolution of turbulence, and an aliasing in measurement. The measurements are limited to the one location where the wire is positioned, and do not provide area coverage. Additionally, data evaluation of the hot wire measurements must make assumptions regarding the isotropy of the flow. The most often used assumption of isotropic flow is generally inaccurate; in most situations of interest, the flow is anisotropic.
To address some of these problems, optical techniques have been developed to non-invasively monitor fluid flow. The optical techniques are based upon the phenomenon that the refractive index of a material varies dependent upon the turbulence. This phenomenon is explained by the nature of turbulence, which is a localized compression and decompression, which effects a refractive index change in the material. Thus a light beam passing through the fluid will be deflected (refracted) in an amount dependent upon the turbulence.
It is known that a single, extremely narrow beam may be used to observe turbulence. The deflection of the laser beam may be recorded in real time. However, in order to effectively use the deflection information, a cumbersome deconvolution of the data is required. An additional drawback of the narrow beam method is that it provides information regarding only the narrow line of turbulence through which it passes; the whole picture is unavailable as in the hot wire technique.
To obtain area coverage of turbulent flow, rather than local coverage, it has been suggested to use a larger beam. However, with a larger beam, the deflection information is not available directly. The larger beam includes deflection information from many different rays with different deflections; this information is encoded in the beam in such a manner that it cannot be viewed directly. Furthermore, the intensity of the straight, undeflected rays is typically many orders of magnitude greater than the intensity of the deflected rays which are of interest.
Spatial filter techniques have been used to recover a part of the deflection information. First, the encoded laser beam is focused by a lens. Using Schlieren techniques at the focal plane, the central portion of the beam is physically blocked by a knife edge or a bull's eye. Some of the deflection information is found in the areas surrounding the blocked portion, and thus this information is not blocked. Thus, the spatially filtered beam contains deflection information which can be used to provide a limited measurement of the turbulence in the fluid. The beam may be imaged to provide a "picture" of the turbulence, or the focal plane may be viewed directly to provide information regarding the spectral density of the turbulence.
A significant problem with blocking is the loss of information. Many deflected rays lie within the blocked central region, and this useful information is lost. In fact, the bulk of the turbulent energy has a low wavenumber (2.pi./.lambda.), and therefore it lies within the blocked central region. Even outside the central block, outlying regions are completely or partially obscured; a knife edge blocks all information over more than one-half of the beam, and a bull's eye has support struts which block a portion of the information of interest. Thus, the physical block greatly reduces the intensity of the desired rays (the "signal"), resulting in a substantial loss of precision and a low signal-to-noise ratio.
None of the prior art systems provide an accurate picture of the turbulent spectrum in two dimensions, and none provide any information about turbulent energies with a low wavenumber. Furthermore, none of the prior art systems can accurately measure an anisotropic fluid flow; i.e., a flow whose spectral density function is different along a first axis than along the axis perpendicular to the first. It would be an advantage to provide an apparatus and method for non-invasively monitoring anisotropic turbulent fluid flow, and providing a two dimensional picture of the turbulent spectrum of that fluid flow, including the portion of the spectrum with low wavenumber.